Optical fiber with reduced attenuation loss

ABSTRACT

An optical fiber having an internal glass portion, a first coating layer surrounding the glass portion and a second coating layer surrounding the first coating layer. The first coating layer is formed from a cured polymeric material obtained by curing a radiation curable composition having a radiation curable oligomer having a backbone derived from polypropylene glycol and a dimer acid based polyester polyol. The cured polymeric material has: (a) a hardening temperature (Th) from −10° C. to about −20° C. and a modulus measured at the Th lower than 5.0 MPa; or (b) a hardening temperature (Th) from −20° C. to about −30° C. and a modulus measured at the Th lower than 20.0 MPa; or (c) a hardening temperature (Th) lower than about −30° C. and a modulus measured at the Th lower than 70.0 MPa.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national phase application based onPCT/EP02/04512, filed Apr. 24, 2002, the content of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an optical fiber having a reducedattenuation of the transmitted signal.

The invention was developed under a joint research agreement betweenPirelli Cavi S. p. A. and DSM Desotech B. V.

BACKGROUND ART

Optical fibers commonly consist of a glass portion (typically with adiameter of about 120-130 μm), inside which the transmitted opticalsignal is confined, The glass portion is typically protected by an outercoating, typically of polymeric material. This protective coatingtypically comprises a first coating layer positioned directly onto theglass surface, also known as the “primary coating”, and of at least asecond coating layer, also known as “secondary coating”, disposed tosurround said first coating. In the art, the combination of primarycoating and secondary coating is sometimes also identified as “primarycoating system”, as both these layer are generally applied during thedrawing manufacturing process of the fiber, in contrast with “secondarycoating layers” which may be applied subsequently. In this case, thecoating in contact with the glass portion of the fiber is called “innerprimary coating” while the coating on the outer surface of the fiber iscalled “outer primary coating”. In the present description and claims,the two coating layers will be identified as primary and secondarycoating, respectively, and the combination of the two as “coatingsystem”.

The thickness of the primary coating typically ranges from about 25 μmto about 35 μm, while the thickness of the secondary coating typicallyranges from about 10 μm to about 30 μm.

These polymer coatings may be obtained from compositions comprisingoligomers and monomers that are generally crosslinked by means of UVirradiation in the presence of a suitable photo-initiator. The twocoatings described above differ, inter alia, in the mechanicalproperties of the respective materials. As a matter of fact whereas thematerial which forms the primary coating is a relatively soft material,with a relatively low modulus of elasticity at room temperature, thematerial which forms the secondary coating is relatively harder, havinghigher modulus of elasticity values at room temperature. The coatingsystem is selected to provide environmental protection to the glassfiber and resistance, inter alia, to the well-known phenomenon ofmicrobending, which can lead to attenuation of the signal transmissioncapability of the fiber and is therefore undesirable. In addition,coating system is designed to provide the desired resistance to physicalhandling forces, such as those encountered when the fiber is submittedto cabling operations.

The optical fiber thus composed usually has a total diameter of about250 μm. However, for particular applications, this total diameter mayalso be smaller; in this case, a coating of reduced thickness isgenerally applied.

In addition, as the operator must be able to identify different fiberswith certainty when a plurality of fibers are contained in the samehousing, it is convenient to color the various fibers with differentidentifying colors. Typically, an optical fiber is color-identified bysurrounding the secondary coating with a third colored polymer layer,commonly known as “ink”, having a thickness typically of between about 2μm and about 10 μm, or alternatively by introducing a colored pigmentdirectly into the composition of the secondary coating.

Among the parameters which characterize primary and secondary coatingsperformances, elastic modulus and glass transition temperature of thecross-linked materials are those which are generally used to define themechanical properties of the coating. When referring to the elasticmodulus it should be clarified that in the patent literature this issometimes referred to as “shear” modulus G (or modulus measured inshear), while in some other cases as “tensile” modulus E′ (or modulusmeasured in tension). The determination of said elastic moduli can bemade by means of DMA (Dynamic mechanical analysis) which is a thermalanalysis technique that measures the properties of the materials as theyare deformed under periodical stress. For polymeric materials, the ratiobetween the two moduli is generally 1:3, i.e. the tensile modulus of apolymeric material is typically about three times the shear modulus (seefor instance the reference book Mechanical Properties and Testing ofPolymers, pp. 183-186; Ed. G. M. Swallowe)

Examples of coating systems are disclosed, for instance, in U.S. Pat.No. 4,962,992. In said patent, it is stated that a soft primary coatingis more likely to resist to lateral loading and thus to microbending. Itthus teaches that an equilibrium shear modulus of about 70-200 psi(0.48-1.38 MPa) is acceptable, while it is preferred that such modulusbeing of 70-150 psi (0.48-1.03 MPa). These values correspond to atensile modulus E′ of 1.4-4.13 MPa and 1.4-3.1 MPa, respectively. Asdisclosed in said patent, a too low equilibrium modulus may cause fiberbuckling inside the primary coating and delamination of the coatingsystem. In addition, said patents suggests that the glass transitiontemperature (Tg) of the primary coating material should not exceed −40°C., said Tg being defined as the temperature, determined by means ofstress/strain measurement, at which the modulus of the material changesfrom a relatively high value occurring in the lower temperature, glassystate of the material to a lower value occurring in the transitionregion to the higher temperature, elastomeric (or rubbery) state of thematerial.

Other examples of coating compositions are disclosed, for instance, inWO 01/05724, which discloses radiation curable fiber optic coatingmaterials comprising a (meth)acrylate urethane compound derived from apolypropylene glycol or comprising a (meth)acrylate urethane compoundderived from a polypropylene glycol and a further polyol including apolyester polyol. These compositions may be used, once cured, as coatingmaterial for optical fibers and optical fiber ribbons, including primarycoatings, secondary coatings, coloured secondary coatings, inks, matrixmaterials and bundling materials. In the introductory part, saiddocument mentions that primary coatings should in particular have a verylow Tg.

However, as noticed by the Applicant, although a primary coating has arelatively low value of Tg (as generally required by the art), the valueof the modulus of the coating material may nevertheless begin toincrease at temperatures much higher than the Tg, typically alreadyabove 0° C. Thus, while a low value of Tg simply implies that thetransition of said coating from its rubbery to its glassy state takesplace at relatively low temperatures, no information can be derived asto which would be the variation of the modulus upon temperaturedecrease. As a matter of fact, an excessive increase of the modulus ofthe primary coating upon temperature decrease may negatively affect theoptical performances of the optical fiber, in particular at the lowtemperature values, thus causing undesirable attenuation of thetransmitted signal due to microbending.

Thus, as observed by the applicant, what seems important for controllingthe microbending of an optical fiber is the temperature at which thecoating material begins the transition from its rubbery state (soft) toits glassy state (hard), which temperature will be referred in thefollowing of this specification and claims as the “hardeningtemperature” of the material, or Th. In particular, attention should bepaid to select a composition which still shows a relatively low modulusat said Th, so that an excessive increase of the modulus upon furthertemperature decrease can be avoided.

In the present description and claims, the term “modulus” is referred tothe modulus of a polymeric material as determined by means of a DMA testin tension, as illustrated in detail in the test method section of theexperimental part of the present specification.

In the present description and claims, the term “hardening temperature”is referred to the transition temperature at which the material shows anappreciable increase of its modulus (upon temperature decrease), thusindicating the beginning of an appreciable change from a relatively softand flexible material (rubber-like material) into a relatively hard andbrittle material (glass-like material). The mathematical determinationof Th will be explained in detail in the following of the description.

According to the present invention, the Applicant has thus found thatattenuation losses caused by microbending onto a coated optical fibers,particularly at the low exercise temperatures, can be reduced bysuitably controlling the increase of the modulus at the lowtemperatures. In particular, the Applicant has found that saidmicrobending losses can be reduced by using a polymeric material for theprimary coating having a low hardening temperature and a comparativelylow modulus at said temperature. In addition, the Applicant has foundthat by selecting coating compositions having a relatively lowequilibrium modulus, said attenuation losses can be further controlledover the whole operating temperature range.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention relates to an opticalfiber comprising an internal glass portion, a first coating layerdisposed to surround said glass portion and a second coating layerdisposed to surround said first coating layer, wherein said firstcoating layer is formed from a cured polymeric material obtained bycuring a radiation curable composition comprising a, radiation curableoligomer comprising a backbone derived from polypropylene glycol and adimer acid based polyester polyol, said cured polymeric material having:

-   -   a) a hardening temperature (Th) of from −10° C. to about −20° C.        and a modulus measured at said Th of less than 5.0 MPa; or    -   b) a Th of from −20° C. to about −30° C. and a modulus measured        at said Th of less than 20.0 MPa; or    -   c) a Th lower than about −30° C. and a modulus measured at said        Th of less than 70.0 MPa.

Preferably said material forming said coating layer has:

-   -   a) a Th of from −10° C. to about −20° C. and a modulus measured        at said Th of less than 4.0 MPa; or    -   b) a Th of from −20° C. to about −30° C. and a modulus measured        at said Th of less than 15.0 MPa; or    -   c) a Th lower than about −30° C. and a modulus measured at said        Th of less than 50.0 MPa.

Preferably, the equilibrium modulus of said polymeric material is lowerthan about 1.5 MPa, more preferably lower than about 1.4 MPa, much morepreferably lower than about 1.3 MPa.

According to a preferred embodiment the glass transition temperature ofthe material is not higher than about −30° C., more preferably nothigher than −40° C. and much more preferably not higher than −50° C.

Preferably, a standard single optical fiber according to the inventionshows a microbending sensitivity at 1550 nm at a temperature of −30° C.of less than 1.5 (dB/km)(g/mm) more preferably of less than 1.2(dB/km)(g/mm), even more preferred less than 1.0 (dB/km)(g/mm), and mostpreferred, less than 0.8 (dB/km)(g/mm), when subjected to the expandabledrum microbending test.

The term standard single mode fiber refers herein to optical fibershaving a refractive index profile of the step-index kind, i.e. a singlesegment profile, with a single variation of the refractive index of0.2%-0.4%, adore radius of about 4.0-4.5 μm and a MAC value of about7.8-8.6.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-section of an optical fiber according tothe invention;

FIG. 2 shows an illustrative DMA plot of a polymeric material for anoptical fiber according to the invention;

FIG. 3 shows the curve corresponding to the first derivative of the DMAplot of FIG. 2;

FIGS. 4 a to 4 c show the experimental DMA plots of three primarycoating materials suitable for an optical fiber according to theinvention;

FIG. 5 shows the experimental DMA plot of a prior art primary coatingmaterial.

FIG. 6 shows an illustrative embodiment of a drawing tower formanufacturing an optical fiber according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

As shown in FIG. 1, an optical fiber according to the inventioncomprises an internal glass portion 101, a first polymeric coating layer102, also known as primary coating, disposed to surround said glassportion and a second polymeric coating layer 103, also known assecondary coating, disposed to surround said first polymeric layer.

As mentioned above, an optical fiber according to the present inventioncomprises a primary coating layer formed from a polymeric materialhaving a relatively low hardening temperature and a correspondingly lowmodulus at said temperature.

To better explain the meaning of the hardening temperature, reference ismade to the curve shown in FIG. 1. This curve, typically obtained by aDMA (Dynamic Mechanical Analysis), represents the variation of themodulus of a polymeric material vs. temperature. As shown by this curve,the polymeric material has a relatively high value of modulus at the lowtemperatures (glassy state, portion “a” of the while said value becomesmuch lower when the polymer is in its rubbery state, at the highertemperatures (portion “b” of the curve, equilibrium modulus). Theoblique portion “d” of the curve represents the transition of thematerial from the glassy to the rubbery state. The transition betweenthe glassy state and the rubbery state is known in the art as the “glasstransition” of the material and is generally associated to a specifictemperature (Tg, glass transition temperature). As apparent from thecurve, the transition between the glassy and the rubbery state takesplace over a relatively wide range of temperatures. For apparentpractical reasons, methods has thus been developed for determining aspecific Tg value for each polymer. One of this methods (see forinstance P. Haines, “Thermal Methods of Analysis”, p. 133. BlackieAcademic and professionals ed.), which is the one used for determiningthe Tg values indicated in the present description and claims, comprisesdetermining the intersection point of two lines. The first line(identified as “A” in FIG. 2) is determined by interpolating the pointsof the DMA curve in the plateau region of the glassy state (portion “a”of the curve). In the practice, for primary coating compositions theinterpolation is calculated for the points in the region from −60° C. to−80° C. The second line (identified as “D” in FIG. 2) is determined asthe tangent to the inflection point of the DMA curve in the obliqueportion “d” of said curve. The inflection point and the inclination ofthe tangent in that point can be determined as usual by means of thefirst derivative of the DMA curve, as shown in FIG. 3. According to thecurve shown in FIG. 3, the abscissa of the minimum point of the curvegives the respective abscissa of the inflection point on the DMA curveof FIG. 2, while the ordinate gives the inclination (angularcoefficient) of the tangent line in said inflection point.

In the practice, the derivative of each experimental point is firstcalculated and then the curve interpolating the derivative points isdetermined as known in the art. For avoiding unnecessary calculations,only those points falling within a relatively narrow temperature rangearound the minimum point are taken into account for the regression.Depending from the distribution of the experimental points, this rangemay vary between 40° C. (about ±20° C. around the minimum point) and 60°C. (about ±20° C. around the minimum point). A 6^(th) degree polynomialcurve is considered in general sufficient to obtain an curve to fit withthe derivative of the experimental points.

As shown in FIG. 2 the so determined glass transition temperature is ofabout −62° C.

Similarly to the Tg, also the hardening temperature (Th) of a polymericmaterial can be determined by the above method. The Th is thusdetermined as the intersection point between line “B” and the abovedefined line “D”, as shown in FIG. 2. Line “B” is determined byinterpolating the points of the DMA curve in the plateau region of therubbery state (portion “b” of the curve) i.e. at the equilibrium modulusof the material. In the practice, for primary coating compositions theinterpolation is calculated for the points in the region from 20° C. and40° C.

As shown in FIG. 2, the Th calculated according to the above method willthus be of about −13° C.

As observed by the Applicant, when the cured material forming theprimary coating of the optical fiber has a Th lower than about −10° C.and a modulus lower than 5.0 MPa, preferably lower than about 4.0 MPa,at said temperature, the optical performance of the optical fiber can beimproved, particularly by reducing its microbending sensitivity,particularly at the low temperatures of exercise, e.g. below 0° C. As amatter of fact, the combination of these two parameters in a curedpolymeric material applied as primary coating on an optical fiberaccording to the invention results in a relatively smooth increase ofthe modulus upon temperature decrease, thus allowing to control themicrobending phenomena down to the lower operating temperature limits,typically −30° C. As further observed by the Applicant, analogouscontrol of the microbending phenomena can be achieved also when thecured polymeric material has a Th lower than −20° C. and a modulus atsaid temperature lower than 20 MPa, preferably lower than 15 MPa, orwhen the cured polymeric material has a Th lower than −30° C. and amodulus at said temperature lower than 70 MPa, preferably lower than 50MPa.

The Applicant has further observed that if the equilibrium modulus ofsaid primary coating is lower than about 1.5 MPa, preferably lower thatabout 1.4 MPa, more preferably lower than 1.3 MPa, the microbendingsensitivity of the fiber can be further reduced, not only at the lowertemperatures of the operating range, but also at higher temperatures,e.g. at the room temperature. Said modulus should however preferably benot lower than about 0.5 M Pa, more preferably not lower than 0.8 MPa inorder not to negatively affect other properties of the fiber.Furthermore, the glass transition temperature of the cured polymericmaterial applied as primary coating on an optical fiber according to theinvention is preferably not higher than about −30° C., more preferablynot higher than −40° C. and much more preferably not higher than −50° C.

All the above indicated parameters, i.e. modulus, Th and Tg can bedetermined by subjecting a polymeric material to a DMA in tensionperformed according to the methodology illustrated in the experimentalpart of the present specification, and by evaluating the respective DMAplot of the material according to the above defined procedure.

Radiation curable carrier systems which are suitable for forming acomposition to be used as primary coating in an optical fiber accordingto the invention contain one or more radiation-curable oligomers ormonomers (reactive diluents) having at least one functional groupcapable of polymerization when exposed to actinic radiation. Suitableradiation-curable oligomers or monomers are now well known and withinthe skill of the art. Commonly, the radiation-curable functionality usedis ethylenic unsaturation, which can be polymerized preferably throughradical polymerization. Preferably, at least about 80 mole %, morepreferably, at least about 90 mole %, and most preferably substantiallyall of the radiation-curable functional groups present in the oligomerare acrylate or methacrylate. For the sake of simplicity, the term“acrylate” as used throughout the present application covers bothacrylate and methacrylate functionality.

A primary coating for an optical fiber according to the presentinvention is made from a radiation curable coating composition icomprising a radiation curable oligomer, said oligomer comprising abackbone derived from polypropylene glycol and a dimer acid basedpolyester polyol. Preferably, the oligomer is a urethane acrylateoligomer comprising said backbone, more preferably a wholly aliphaticurethane acrylate oligomer.

The oligomer can be made according to methods that are well known in theart. Preferably, the urethane acrylate oligomer can be prepared byreacting

(A1) the polypropylene glycol, and

(A2) the dimer acid based polyester polyol,

(B) a polyisocyanate, and

(C) a (meth)acrylate containing a hydroxyl group.

Given as examples of the process for manufacturing the urethane acrylateby reacting these compounds are

(i) reacting said glycol (A1 and A2), the polyisocyanate, and thehydroxyl group-containing (meth)acrylate altogether; or

(ii) reacting said glycol and the polyisocyanate, an reacting theresulting product with the hydroxyl group-containing (meth)acrylate; or

(iii) reacting the polyisocyanate and the hydroxyl group-containing(meth)acrylate, and reacting the resulting product with said glycol; or

(iv) reacting the polyisocyanate and the hydroxyl group-containing(meth)acrylate, reacting the resulting product with said glycol, andreacting the hydroxyl group-containing (meth)acrylate once more.

Polypropylene glycol (A1)—as used herein—is understood to refer to apolypropylene glycol comprising composition having a plurality ofpolypropylene glycol moieties. Preferably, said polypropylene glycol hason average a number average molecular weight ranging from 1,000 to13,000, more preferably ranging from 1,500 to 8,000, even more preferredfrom 2,000 to 6,000, and most preferred from 2,500 to 4,500. Accordingto a preferred embodiment, the amount of unsaturation (referred to themeq/g unsaturation for the total composition) of said polypropyleneglycol is less than 0.01 meq/g, more preferably between 0.0001 and 0.009meq/g.

Polypropylene glycol includes 1,2-polypropylene glycol,1,3-polypropylene glycol and mixtures thereof, with 1,2-polypropyleneglycol being preferred. Suitable polypropylene glycols are commerciallyavailable under the trade names of, for example, Voranol P1010, P 2001and P 3000 (supplied by Dow), Lupranol 1000 and 1100 (supplied byElastogran), ACCLAIM 2200, 3201, 4200, 6300, 8200, and Desmophen 1111BD, 1112 BD, 2061 BD, 2062 BD (all manufactured by Bayer), and the like.Such urethane compounds may be formed by any reaction technique suitablefor such purpose.

Dimer acid based polyester polyol (A2)—as used herein—is understood torefer to a hydroxyl-terminated polyester polyol which has been made bypolymerizing an acid component and a hydroxyl-component and which hasdimer acid residues in its structure, wherein said dimer acid residuesare residues derived from the use of a dimer acid as at least part ofthe acid-component and/or by the use of the diol derivative of a dimeracid as at least part of the hydroxyl-component.

Dimer acids (and esters thereof) are a well known commercially availableclass of dicarboxylic acids (or esters). They are normally prepared bydimerizing unsaturated long chain aliphatic monocarboxylic acids,usually of 13 to 22 carbon atoms, or their esters (e.g. alkyl esters).The dimerization is thought by those in the art to proceed by possiblemechanisms which include Diels-Alder, free radical, and carbonium ionmechanisms. The dimer acid material will usually contain 26 to 44 carbonatoms. Particularly, examples include dimer acids (or esters) derivedfrom C-18 and C-22 unsaturated monocarboxylic acids (or esters) whichwill yield, respectively, C-36 and C-44 dimer acids (or esters). Dimeracids derived from C-18 unsaturated acids, which include acids such aslinoleic and linolenic are particularly well known (yielding C-36 dimeracids).

The dimer acid products will normally also contain a proportion oftrimer acids (e.g. C-54 acids when using C-18 starting acids), possiblyeven higher oligomers and also small amounts of the monomer acids.Several different grades of dimer acids are available from commercialsources and these differ from each other primarily in the amount ofmonobasic and trimer acid fractions and the degree of unsaturation.

Usually the dimer acid (or ester) products as initially formed areunsaturated which could possibly be detrimental to their oxidativestability by providing sites for crosslinking or degradation and soresulting in changes in the physical properties of the coating filmswith time. It is therefore preferable (although not essential) to usedimer acid products which have been hydrogenated to remove a substantialproportion of the unreacted double bonds.

Herein the term “dimer acid” is used to collectively convey both thediacid material itself or ester-forming derivatives thereof (such aslower alkyl esters) which would act as an acid component in polyestersynthesis and includes (if present) any trimer or monomer.

The dimer acid based polyester polyol preferably has on average a numberaverage molecular weight ranging from 1,000 to 13,000, more preferablyranging from 1,500 to 8,000, even more preferred from 2,000 to 6,000,and most preferred from 2,500 to 4,000.

Examples of these dimer acid based polyester polyols are given in EP 0539 030 B1 which polyols are incorporated herein by reference. Ascommercially available products, Priplast 3190, 3191, 3192, 3195, 3196,3197, 3198, 1838, 2033 (manufactured by Uniqema), and the like can begiven.

The ratio of polypropylene glycol to dimer add based polyester polyol inthe oligomer may be ranging from 1:5 to 5:1, preferably ranging from 1:4to 4:1, and more preferably ranging from 1:2 to 2:1, even morepreferably, polypropylene glycol and dimer acid based polyester polyolare present in an equimolar ratio.

Given as examples of the polyisocyanate (B) are 2,4-tolylenediisocyanate, 2,6-tolylene diisocyanate, 1,3-xylylene diisocyanate,1,4-xylylene diisocyanate, 1,5-naphthalene diisocyanate, m-phenylenediisocyanate, p-phenylene diisocyanate,3,3′-dimethyl-4,4′-diphenylmethane diisocyanate, 4,4′-diphenylmethanediisocyanate, 3,3′-dimethylphenylene diisocyanate, 4,4′-biphenylenediisocyanate, 1,6-hexane diisocyanate, isophorone diisocyanate,methylenebis(4-cyclohexylisocyanate), 2,2,4-trimethylhexamethylenediisocyanate, bis(2-isocyanatoethyl)fumarate, 6-isopropyl-1,3-phenyldiisocyanate, 4-diphenylpropane diisocyanate, hydrogenateddiphenylmethane diisocyanate, hydrogenated xylylene diisocyanate,tetramethyl xylylene diisocyanate, lysine isocyanate, and the like.These polyisocyanate compounds may be used either individually or incombinations of two or more. Preferred isocyanates are tolylenedi-isocyanate, isophorone di-isocyanate, and methylene-bis(4-cyclohexylisocyanate). Most preferred are wholly aliphatic basedpolyisocyanate compounds, such as isophorone di-isocyanate, andmethylene-bis (4-cyclohexylisocyanate).

Examples of the hydroxyl group-containing acrylate (C) include,(meth)acrylates derived from (meth)acrylic acid and epoxy and(meth)acrylates comprising alkylene oxides, more in particular,2-hydroxyethyl(meth)acrylate, 2-hydroxypropylacrylate and2-hydroxy-3-oxyphenyl(meth)acrylate. Acrylate functional groups arepreferred over methacrylates.

The ratio of the polyol (A) [said polyol (A) comprising (A1) and (A2)],the polyisocyanate (B), and the hydroxyl group-containing acrylate (C)used for preparing the urethane acrylate is determined so that 1.1 to 3equivalents of an isocyanate group included in the polyisocyanate and0.1 to 1.5 equivalents of a hydroxyl group included in the hydroxylgroup-containing (meth)acrylate are used for one equivalent of thehydroxyl group included in the polyol.

The number average molecular weight of the urethane (meth)acrylateoligomer used in the composition of the present invention is preferablyin the range from 1200 to 20,000, and more preferably from 2,200 to10,000. If the number average molecular weight of the urethane(meth)acrylate is less than 100, the resin composition tends tosolidify; on the other hand, if the number average molecular weight islarger than 20,000, the viscosity of the composition becomes high,making handling of the composition difficult.

The urethane (meth)acrylate oligomer is preferably used in an amountfrom 10 to 90 wt %, more preferably from 20 to 80 wt %, even morepreferably from 30 to 70 wt. %, and most preferred from 40 to 70 wt. %of the total amount of the resin composition. When the composition isused as a coating material for optical fibers, the range from 20 to 80wt. % is particularly preferable to ensure excellent coatability, aswell as superior flexibility and long-term reliability of the curedcoating.

A radiation-curable composition to be applied as a primary coating on anoptical fiber according to the invention may also contain one or morereactive diluents (B) that are used to adjust the viscosity. Thereactive diluent can be a low viscosity monomer having at least onefunctional group capable of polymerization when exposed to actinicradiation. This functional group may be of the same nature as that usedin the radiation-curable oligomer. Preferably, the functional group ofeach reactive diluent is capable of copolymerizing with theradiation-curable functional group present on the otherradiation-curable diluents or oligomer. The reactive diluents used canbe mono- and/or multifunctional, preferably (meth)acrylate functional.

A suitable radiation-curable primary coating composition comprises fromabout 1 to about 80 wt. % of at least one radiation-curable diluent.Preferred amounts of the radiation-curable diluent include from about 10to about 60 wt. %, more preferably from about 20 to about 55 wt. %, evenmore preferred ranging from 25 to 40 wt. %, based on the total weight ofthe coating composition.

Generally, each reactive diluent has a molecular weight of less thanabout 550 and a viscosity of less than about 500 mPa.s

For example, the reactive diluent can be a monomer or a mixture ofmonomers having an acrylate or vinyl ether functionality and a C₄-C₂₀alkyl or polyether moiety. Examples of acrylate functionalmonofunctional diluents are acrylates containing an alicyclic structuresuch as isobornyl acrylate, bornyl acrylate, dicyclopentanyl acrylate,cyclohexyl acrylate, and the like, 2-hydroxyethyl acrylate,2-hydroxypropyl acrylate, 2-hydroxybutyl acrylate, methyl acrylate,ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate,amyl acrylate, isobutyl acrylate, t-butyl acrylate, pentyl acrylate,isoamyl acrylate, hexyl acryl ate, heptyl acrylate, octyl acrylate,isooctyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, decylacrylate, isodecyl acrylate, undecyl acrylate, dodecyl acrylate, laurylacrylate, stearyl acrylate, isostearyl acrylate, tetrahydrofurfurylacrylate, butoxyethyl acrylate, ethoxydiethylene glycol acrylate,benzylacrylate, phenoxyethylacrylate, polyethylene glycol monoacrylate,polypropylene glycol monoacrylate, methoxyethylene glycol acrylate,ethoxy ethyl acrylate, methoxypolyethylene glycol acrylate,methoxypropylene glycol acrylate, dimethylaminoethyl acrylate,diethylamino ethyl acrylate, 7-amino-3,7-dimethyloctyl acrylate,acrylate monomers shown by the following formula (1),

wherein R⁷ is a hydrogen atom or a methyl group, R⁸ is an alkylene grouphaving 2-6, and preferably 2-4 carbon atoms, R⁹ is a hydrogen atom or anorganic group containing 1-12 carbon atoms or an aromatic ring, and r isan integer from 0 to 12, and preferably from 1 to 8.

Of these, in order to obtain a cured polymeric material having asuitably low hardening temperature and a suitably low modulus at saidtemperature, long aliphatic chain-substituted monoacrylates, such as,for example decyl acrylate, isodecyl acrylate, tridecyl acrylate, laurylacrylate, and the like, are preferred and alkoxylated alkyl phenolacrylates, such as ethoxylated and propoxylated nonyl phenol acrylateare further preferred.

Examples of non-acrylate functional monomer diluents areN-vinylpyrrolidone, N-vinyl caprolactam, vinylimidazole, vinylpyridine,and the like.

These N-vinyl monomers preferably are present in amounts between about 1and about 20% by weight, more preferably less than about 10% by weight,even more preferred ranging from 2 to 7% by weight.

According to a preferred embodiment, the polymeric material applied asprimary coating on an optical fiber according to the invention is madefrom a radiation curable composition comprising at least onemonofunctional reactive diluent (having an acrylate or vinyl etherfunctionality), said monofunctional diluent(s) being present in amountsranging from 10 to 50 wt. %, preferably ranging from 20 to 40 wt. %,more preferably from 25 to 38 wt. %. The amount of mono-acrylatefunctional reactive diluents preferably ranges from 10 to 40 wt. %, morepreferably from 15 to 35 wt. % and most preferred from 20 to 30 wt. %.

The reactive diluent can also comprise a diluent having two or morefunctional groups capable of polymerization. Examples of such monomersinclude: C₂-C₁₈ hydrocarbondiol diacrylates, C₄-C₁₈ hydrocarbondivinylethers,

C₃-C₁₈ hydrocarbon triacrylates, and the polyether analogues thereof,and the like, such, as 1,6-hexanedioldiacrylate, trimethylolpropanetriacrylate, hexanediol divinylether, triethyleneglycol diacrylate,pentaerythritol triacrylate, ethoxylated bisphenol-A diacrylate, andtripropyleneglycol diacrylate.

Such multifunctional reactive diluents are preferably (meth)acrylatefunctional, preferably difunctional (component (B1)) and trifunctional(component (B2)).

Preferably, alkoxylated aliphatic polyacrylates are used, such asethoxylated hexanedioldiacrylate, propoxylated glyceryl triacrylate orpropoxylated trimethylol propane triacrylate.

Preferred examples of diacrylates are alkoxylated aliphatic glycoldiacrylate, more preferably, propoxylated aliphatic glycol diacrylate. Apreferred example of a triacrylate is trimethylol propane triacrylate.

According to a preferred embodiment the polymeric material applied asprimary coating on an optical fiber according to the invention is madefrom a radiation curable which comprises, a multifunctional reactivediluent n amountsranging from 0.5-10 wt. %, more preferably ranging from1 to 5 wt. %, and most preferred from 1.5 to 3 wt. %.

Without being bound to any particular theory, the present inventorsbelieve that the combination of the oligomer according to the presentinvention in amounts of less than about 75 wt. % (preferably less thanabout 70 wt. %) with a total amount of monofunctional reactive diluentsof at least about 15 wt. % (more preferably, at least about 20 wt. %,even more preferably at least about 25 wt. % and most preferred at leastabout 30 wt. %) aids in achieving a primary coating composition, thatafter cure, has an acceptably low hardening temperature and low modulusat said temperature.

It is further preferred that the composition comprises a mixture of atleast two monofunctional reactive diluents, more preferably, one of saidreactive diluents being substituted with a long aliphatic chain; eve nmore preferably, the composition contains two long aliphaticchain-substituted monoacrylates. Preferably, at least about 10 wt. %,more preferably at least about 12 wt. % is present of said at least onelong aliphatic chain-substituted monoacrylate.

A liquid curable resin composition suitable to be applied as a primarycoating layer on an optical fiber according to the present invention canbe cured by radiation. Here, radiation includes infrared radiation,visible rays, ultraviolet radiation, X-rays, electron beams, α-rays,β-rays, γ-rays, and the like. Visible and UV radiation are preferred.

The liquid curable resin composition suitable to be applied as a primarycoating layer on an optical fiber according to the present inventionpreferably comprises a photo-polymerization initiator. In addition, aphotosensitizer can added as required. Given as examples of thephoto-polymerization initiator are 1-hydroxycyclohexylphenyl ketone,2,2-dimethoxy-2-phenylacetophenone, xanthone, fluorenone, benzaldehyde,fluorene, anthraquinone, triphenylamine, carbazole,3-methylacetophenone, 4-chlorobenzophenone, 4,4′-dimethoxybenzophenone,4,4′-diaminobenzophenone, Michler's ketone, benzoin propyl ether,benzoin ethyl ether, benzyl methyl ketal,1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one,2-hydroxy-2-methyl-1-phenylpropan-1-one, thioxanethone,diethylthioxanthone, 2-isopropylthioxanthone, 2-chlorothioxanthone,2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propan-1-one,2,4,6-trimethylbenzoyldiphenylphosphine oxide,bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide,bis-(2,4,6-trimethylbenzoyl)-phenylphosphine oxide and the like.

Examples of commercially available products of the photo-polymerizationinitiator include IRGACURE 184, 369, 651, 500, 907, CGI1700, 1750, 1850,819, Darocur 1116, 1173 (manufactured by Ciba Specialty Chemicals Co.,Ltd.), Lucirin LR8728 (manufactured by BASF), Ubecryl P36 (manufacturedby UCB), and the like.

The amount of the polymerization initiator use can range from 0.1 to 10wt %, and preferably from 0.5 to 7 wt %, of the total amount of thecomponents for the resin composition.

In addition to the above-described components, various additives such asantioxidants, UV absorbers, light stabilizers, silane coupling agents,coating surface improvers, heat polymerization inhibitors, levelingagents, surfactants, colorants, preservatives, plasticizers, lubricants,solvents, fillers, aging preventives, and wettability improvers can beused in the liquid curable resin composition of the present invention,as required. Examples of antioxidants include Irganox 1010, 1035, 1076,1222 (manufactured by Ciba Specialty Chemicals Co., Ltd.), Antigene P,3C, FR, Sumilizer GA-80 (manufactured by Sumitomo Chemical IndustriesCo., Ltd.), and the like; examples of UV absorbers include Tinuvin P,234, 320, 326, 327, 328, 329, 213 (manufactured by Ciba SpecialtyChemicals Co., Ltd.), Seesorb 102, 103, 110, 501., 202, 712, 704(manufactured by Sypro Chemical Co., Ltd.), and the like; examples oflight stabilizers include Tinuvin 292, 144, 622LD (manufactured by CibaSpecialty Chemicals Co., Ltd.), Sanol LS770 (manufactured by Sankyo Co.,Ltd.), Sumisorb TM-061 (manufactured by Sumitomo Chemical IndustriesCo., Ltd.), and the like; examples of silane coupling agents includeaminopropyltriethoxysilane, mercaptopropyltrimethoxy-silane, andmethacryloxypropyltrimethoxysilane, and commercially available productssuch as SH6062, SH6030 (manufactured by Toray-Dow Coming Silicone Co.,Ltd.), and KBE903, KBE603, KBE403 (manufactured by Shin-Etsu ChemicalCo., Ltd.).

The viscosity of the liquid curable resin composition applied as aprimary coating layer on an optical fiber according to the presentinvention is usually in the range from 200 to 20,000 cP, and preferably,from 2,000 to 15,000 cP.

The primary coating compositions suitable to be applied as a primarycoating layer on an optical fiber according present invention, whencured, typically have an elongation-at-break of greater than 80%, morepreferably of at least 110%, more preferably at least 150% but nottypically higher than 400%.

The compositions suitable to be applied as a primary coating layer on anoptical fiber according to the the present invention will preferablyhave a cure speed of 1.0 J/cm² (at 95% of maximum attainable modulus) orless, more preferably about 0.7 J/cm²or less, and more preferably, about0.5 J/cm² or less, and most preferred, about 0.4 J/cm² or less.

An optical fiber according to the invention comprises a second layer ofpolymeric material (secondary coating) which is disposed to surroundsaid primary coating. Preferably, the polymeric material of saidsecondary coating is also based on a radiation curable composition. Theaforedescribed primary coating is then in turn coated with a secondarycoating, of a type known in the art, compatible with the primary coatingformulation. For example, if the primary coating has an acrylic base,the secondary coating will also preferably have an acrylic base.

Typically, an acrylic based secondary coating comprises at least oneoligomer with acrylate or methacrylate terminal groups, at least oneacrylic diluent monomer and at least one photoinitiator.

The oligomer represents generally 40-80% of the formulation by weight.The oligomer commonly consists of a polyurethaneacrylate.

The polyurethaneacrylate is prepared by reaction between a polyolstructure, a polyisocyanate and a monomer carrying the acrylic function.

The molecular weight of the polyol structure is indicatively between 500and 6000 u.a.; it can be entirely of hydrocarbon, polyether polyester,polysiloxane or fluorinated type, or be a combination thereof. Thehydrocarbon and polyether structure and their combinations arepreferred. A structure representative of a polyether polyol can be, forexample, polytetramethylene oxide, polymethyltetra methylene oxide,polymethylene oxide, polypropylene oxide, polybutylene oxide, theirisomers and their mixtures. Structures representative of a hydrocarbonpolyol are polybutadiene or polyisobutylene, completely or partlyhydrogenated and functionalized with hydroxyl groups.

The polyisocyanate can be of aromatic or aliphatic type, such as, forinstance, a polyisocyanate (B) as previously described.

The monomer carrying the acrylic function comprises groups able to reactwith the isocyanic group. Said monomer can be selected, for instance,among the hydroxyl group-containing acrylates (C) as previouslyillustrated.

The epoxyacrylate is prepared by reacting the acrylic acid with aglycidylether of an alcohol, typically bisphenol A or bisphenol F.

The diluent monomer represents 20-50% of the formulation by weight, itsmain purpose being to cause the formulation to attain a viscosity ofabout 5 Pas at the secondary coating application temperature. Thediluent monomer, carrying the reactive function, preferably of acrylictype, has a structure compatible with that of the oligomer. The acrylicfunction is preferred. The diluent monomer can contain an alkylstructure, such as isobornylacrylate, hexanediacrylate,dicyclopentadiene acrylate, trimethylolpropane-triacrylate, or aromaticsuch as nonylphenyletheracrylate, polyethyleneglycol-phenyletheracrylateand acrylic derivatives of bisphenol A.

A photoinitiator, such as those previously illustrated is preferablyaded to the composition. Further additives, such as inhibitorsinhibiting polymerization by the effect of temperature, lightstabilizers, levelling agents and detachment promotors can also beadded.

A typical formulation of a cross-linkable system for secondary coatingscomprises about 40-70% of polyurethaneacrylate, epoxyacrylate or theirmixtures, about 30-50% of diluent monomer, about 1-5% of photoinitiatorand about 0.5-5% of other additives.

An example of a formulation usable as the secondary coating of theinvention is that marketed under the name of DeSolite® 3471-2-136 (DSM).The fibres obtained thereby can be used either as such within opticalcables, or can be combined, for example in ribbon form, by incorporationinto a common polymer coating, of a type known in the art (such asCablelite® 3287-9-53, DSM), to be then used to form an optical cable.

Typically, the polymeric material forming the secondary coating has amodulus E′ at 25° C. of from about 1000 MPa to about 2000 MPa and aglass transition temperature (measured as above defined) higher thanabout 30° C., preferably higher than 40° C. and more preferably higherthan about 50° C.

An optical fiber according to the present invention may be producedaccording to the usual drawing techniques, using, for example, a systemsuch as the one schematically illustrated in FIG. 2.

This system, commonly known as “drawing tower”, typically comprises afurnace (302) inside which a glass optical preform to be drawn isplaced. The bottom part of the said preform is heated to the softeningpoint and drawn into an optical fiber (301). The fiber is then cooled,preferably to a temperature of at least 60° C., preferably in a suitablecooling tube (303) of the type described, for example, in patentapplication WO 99/26891, and passed through a diameter measurementdevice (304). This device is connected by means of a microprocessor (313to a pulley (310) which regulates the spinning speed; in the event ofany variation in the diameter of the fiber, the microprocessor (313)acts to regulate the rotational speed of the pulley (310), so as to keepthe diameter of the optical fiber constant. Then, the fiber passesthrough a primary coating applicator (305), containing the coatingcomposition in liquid form, and is covered with this composition to athickness of about 25 μm-35 μm. The coated fiber is then passed througha UV oven (or a series of ovens) (306) in which the primary coating iscured. The fiber coated with the cured primary coating is then passedthrough a second applicator (307), in which it is coated with thesecondary coating and then cured in the relative UV oven (or series ofovens) (308). Alternatively, the application of the secondary coatingmay be carried out directly on the primary coating before the latter hasbeen cured, according to the “wet-on-wet” technique. In this case, asingle applicator is used, which allows the sequential application ofthe two coating layers, for example, of the type described in patentU.S. Pat. No. 4,474,830. The fiber thus coated is then cured using oneor more UV ovens similar to those used to cure the individual coatings.

Subsequent to the coating and to the curing, the fiber may optionally becaused to pass through a device capable of giving a predeterminedtorsion to this fiber, for example of the type described ininternational patent application WO 99/67180, for the purpose ofreducing the PMD (“polarization Mode Dispersion”) value of this fiber.The pulley (310) placed downstream of the devices illustrated previouslycontrols the spinning speed of the fiber. After this drawing pulley, thefiber passes through a device (311) capable of controlling the tensionof the fiber, of the type described, for example, in patent applicationEP 1 112 979, and is finally collected on a reel (312).

An optical fiber thus produced may be used in the production of opticalcables. The fiber may be used either as such or in the form of ribbonscomprising several fibers combined together by means of a commoncoating.

EXAMPLES

The present invention will be explained in more detail below by way ofexamples, which are not intended to be limiting of the presentinvention.

Coating Compositions

Coating compositions have been prepared to be applied as a primarycoating on optical fibers. The compositions to be applied as a primarycoating on an optical fiber according to the invention are indicated asExamples Ex.1, Ex.2 and Ex.3 in the following table 1.

TABLE 1 Radiation curable primary coating compositions Ex. 1 Ex. 2 Ex. 3(Wt. %) (Wt. %) (Wt. %) Oligomer I 68.30 60.30 67.30 Ethoxylated nonylphenol acrylate 10.00 19.00 10.00 Tridecyl acrylate 10.00 10.00 10.00Long aliphatic chain-substituted 2.00 2.00 2.00 monoacrylate Vinylcaprolactam 5.00 6.00 5.00 Ethoxylated bisphenol A diacrylate 1.00 —3.00 Trimethylol propane triacrylate (TMPTA) 1.00 — —2,4,6-trimethylbenzoyl diphenyl phosphine 1.40 1.40 1.40 oxideThiodiethylene bis[3-(3,5-di-tert-butyl-4- 0.30 0.30 0.30hydroxyphenyl)propionate])hydrocin- namate γ-mercapto propyltrimethoxysilane 1.00 1.00 1.00Oligomer I is the reaction product of isophorone diisocyanate (IPDI),2-hydroxyethylacrylate (HEA), polypropylene glycol (PPG) and a dimeracid based polyester polyol.

In addition, comparative commercial primary coating DeSolite® 3471-1-129has been tested as a comparative experiment (Comp. Exp. A in table 2)has also been tested

The equilibrium modulus, the Tg; the Th and the modulus at the Th foreach of the above cured primary coating compositions were as given inTable 2(see test method section for details on DMA test anddetermination of respective parameters on the DMA curve). Thecorresponding DMA curves of said cured coating compositions are reportedin FIGS. 4A, 4B, 4C (examples 1, 2 and 3), and 5 (comparative experimentA), respectively.

TABLE 2 Parameters of cured primary coating compositions Tg Th E′ E′(Th) Ex. 1 −59.1 −12.2 1.1 3.5 Ex. 2 −56.6 −10.8 0.7 2.0 Ex. 3 −63.2−13.3 1.1 2.7 Comp. A −55.1 −5.6 1.9 3.6

Preparation of Optical Fibers

Coated single mode optical fibers have been manufactured as indicated inthe test method section, by using a primary coating compositions ofExamples 1-3 (corresponding to optical fibers F1, F1a, F2 and F3 intable 3) or of Comparative Experiment A (fiber Fc in table 3).Commercial secondary coating DeSolite® 3471-2-136 has been used for allfibers

The following single mode optical fibers have been manufactured:

Fiber Primary coating MAC F1 Ex. 1 8.0 F1a Ex. 1 7.9 F2 Ex. 2 7.9 F3 Ex.3 8.35 Fc Comp. A 8.23

The MAC value for each fiber is determined as indicated in the testmethod section.

Microbending Tests

The results of the microbending test (see details in the test methodssection) on single mode optical fibers are reported in the followingtable 4.

TABLE 4 Microbending on SM fibers Microbending Sensitivity(dB/Km)/(g/mm) Fiber MAC −30° C. +22° C. +60° C. F1 8.00 0.75 0.4 1.6F1a 7.91 0.45 0.31 1.5 F2 7.9 0.4 0.2 1.3 F3 8.35 0.5 0.3 1.6 Fc 8.231.6 1.4 2.6

As shown by the above results, an optical fiber according to theinvention is less prone to attenuation losses caused by the microbendingphenomenon, both at the low as well as high operating temperatures.

Test Methods and Methods of Manufacturing

Curing of the Primary Coatings for Mechanical Testing (SamplePreparation)

A drawdown of the material to be tested was made on a glass plate andcured using a UV processor in inert atmosphere (with a UV dose of 1J/cm², Fusion D-lamp measured with EIT Uvicure or International Light IL390 B Radio meter). The cured film was conditioned at 23±2° C. and 50±5%RH for a minimumn of 16 hours prior to testing.

A minimum of 6 test specimens having a width of 12.7 mm and a length of12.5 cm were cut from the cured film.

Dynamic Mechanical Testing

The DMA testing has been carried out in tension according to thefollowing methodology.

Test samples of the cured coating compositions of examples 1-3 and ofcomparative experiment A were measured using a Rheometrics SolidsAnalyzer (RSA-11), equipped with:

1) a personal computer having a Windows operating system and having RSIOrchestrator® software (Version V.6.4.1) loaded, and

2) a liquid nitrogen controller system for low-temperature operation.

The test samples were prepared by casting a film of the material, havinga thickness in the range of 0.02 mm to 0.4 mm, on a glass plate. Thesample film was cured using a UV processor. A specimen approximately 35mm (1.4 inches) long and approximately 12 mm wide was cut from adefect-free region of the cured film. For soft films, which tend to havesticky surfaces, a cotton-tipped applicator was used to coat the cutspecimen with talc powder.

The film thickness of the specimen was measured at five or morelocations along the length. The average film thickness was calculated to±0.001 mm. The thickness cannot vary by more than 0.01 mm over thislength. Another specimen was taken if this condition was not met. Thewidth of the specimen was measured at two or more locations and theaverage value calculated to ±0.1 mm.

The geometry of the sample was entered into the instrument. The-lengthfield was set at a value of 23.2 mm and the-measured values of width andthickness of the sample specimen were entered into the appropriatefields.

Before, conducting the temperature sweep, moisture was removed from thetest samples by subjecting the test samples to a temperature of 80° C.in a nitrogen atmosphere for 5 minutes. The temperature sweep usedincluded cooling the test samples to about −60° C. or about −90° C. andincreasing the temperature at about 2° C./minute until the temperaturereached about 100° C. to about 120° C. The test frequency used was 1.0radian/second. In a DMTA measurement, which is a dynamic measurement,the following moduli are measured: the storage modulus. E′ (alsoreferred to as elastic modulus), and the loss modulus E″ (also referredto as viscous modulus). The lowest value of the storage modulus E′ inthe DMTA curve in the temperature range between 10 and 100° C. measuredat a frequency of 1.0 radian/second under the conditions as described indetail above is taken as the equilibrium modulus of the coating.

The corresponding DMA curves are reported in FIGS. 4 a to 4 c (examples1-3 respectively) and FIG. 5 (comp. Exp. A).

Determination of Glass Transition Temperature (Tg) and HardeningTemperature (Th)

Based on the respective DMA plot of each cured primary coating material,the Tg, Th and modulus at Th of the material have been determined asmentioned in the descriptive part.

Thus, with ref. to FIG. 1, the Tg is determined by the intersectionpoint of line A with line D. Line A is determined by interpolating thepoints of the DMA curve in the plateau region of the glassy statein thefollowing manner. First of all, the median value of logE′ in the regionfrom −60° C. to −80° C. is calculated. Line A is then determined as thehorizontal line (parallel to the x axis) passing through said value ofLogE′. Line D is determined as the tangent to the inflection point ofthe DMA curve in the oblique portion “d” of said curve. The inflectionpoint and the inclination of the tangent in that point are determined bymeans of the first derivative of the DMA curve; the abscissa of theminimum point of the derivative curve gives the respective abscissa ofthe inflection point on the DMA curve, while the ordinate gives theinclination (angular coefficient) of the tangent line in said inflectionpoint. The derivative curve has been determined by calculating thederivative of each experimental point of the DMA curve and then fittingthese points by means of a 6^(th) degree polynomial curve in the range±20/−40° C. around the minimum calculated derivative points.

Similarly, also the Th has been determined as the intersection point ofline B with line D(see FIG. 1). Une D is as above determined, while lineB is determined by interpolating the points of the DMA curve in theplateau region of the rubbery state in the following manner. First ofall, the median value of logE′ in the region from 20° C. to 40° C. iscalculated. Line B is then determined as the horizontal line (parallelto the x axis) passing through said median value of LogE′.

Manufacturing of Optical Fibers

All the optical fibers used in the present experimental section has beenmanufactured according to standard drawing techniques, by applying afirst (primary) coating composition on the drawn optical fiber, curingsaid coating composition and subsequently applying the secondary coatinglayer and curing it. The fiber is drawn at a speed of about 20 m/s andthe cure degree of the coating layers is of at least 90%. The curedegree is determined by means of MICRO-FTIR technique, by determiningthe percentage of the reacted acrylate instaurations in the finalcross-linked resin with respect to the initial photo-curable composition(e.g. as described in WO 98/50317).

Microbending Tests

Microbending effects on optical fibers were determined by the“expandable drum method” as described, for example, in G. Grasso and F.Meli “Microbending losses of cabled single-mode fibers”, ECOC '88 pp526-ff, or as defined by IEC standard 62221 (Optical fibers—Measurementmethods—Microbending sensitivity—Method A, Expandable drum; October2001). The test is performed by winding a 100 m length fiber with atension of 55 g on a 300 mm diameter expandable metallic bobbin, coatedwith rough material (3M Imperial® PSA-grade 40 μm).

The bobbin is connected with a personal computer which controls:

-   the expansion of the bobbin (in terms of variation of fiber length);    and-   the fiber transmission loss.

The bobbin is then gradually expanded while monitoring fibertransmission loss versus fiber strain.

The pressure exerted onto the fiber is calculated from the fiberelongation by the following formula:

$p = \frac{E\; A\; ɛ}{R}$

where E is the elastic modulus of glass, A the area of the coated fiberand R the bobbin radius.

For each optical fiber, the MAC has been determined as follows:

${MAC} = \frac{MFD}{\lambda_{co}}$where MFD (mode field diameter according Petermann definition) at 1550nm and λ_(co) (lambda fiber cutoff−2 m length) are determined accordingto standard ITUT G650.

1. An optical fiber comprising an internal glass portion, a firstcoating layer surrounding said glass portion and a second coating layersurrounding said first coating layer, wherein said first coating layeris formed from a cured polymeric material obtained by curing a radiationcurable composition comprising a radiation curable oligomer comprising abackbone derived from polypropylene glycol and a dimer acid basedpolyester polyol, said cured polymeric material having: a) a hardeningtemperature (Th) from −10° C. to about −20° C. and a modulus measured atsaid Th lower than 5.0 MPa; or b) a hardening temperature (Th) from −20°C. to about −30° C. and a modulus measured at said Th lower than 20.0MPa; or c) a hardening temperature (Th) lower than about −30° C. and amodulus measured at said Th lower than 70.0 MPa.
 2. The optical fiberaccording to claim 1, wherein said material forming said coating layerhas: a) a hardening temperature (Th) from −10° C. to about −20° C. and amodulus measured at said Th lower than 4.0 MPa; or b) a hardeningtemperature (Th) from −20° C. to about −30° C. and a modulus measured atsaid Th lower than 15.0 MPa; or c) a hardening temperature (Th) lowerthan about −30° C. and a modulus measured at said Th lower than 50.0MPa.
 3. The optical fiber according to claim 1, wherein said polymericmaterial has an equilibrium modulus lower than about 1.5 MPa.
 4. Theoptical fiber according to claim 1, where said polymeric material has anequilibrium modulus lower than about 1.4 MPa.
 5. The optical fiberaccording to claim 1, wherein said polymeric material has an equilibriummodulus lower than about 1.3 MPa.
 6. The optical fiber according toclaim 1, wherein the polymeric material has a glass transitiontemperature not higher than about −30° C.
 7. The optical fiber accordingto claim 1, wherein the polymeric material has a glass transitiontemperature not higher than about −40° C.
 8. The optical fiber accordingto claim 1, wherein the polymeric material has a glass transitiontemperature not higher than about −50° C.